Put the beef in a large pan and cover with cold water. Add the celery and onions, season with 1 teaspoon of sea salt and a generous pinch of black pepper, then gently bring to the boil. Reduce to a simmer, cover with a lid and leave to tick away for 2 hours.
After 1 hour 10 minutes, add the carrots and corn and simmer for about 15 minutes, or until the veg are cooked. Remove the veg from the pan with a slotted spoon and set aside.
Add the sweet potatoes to the pan and cook for 15 minutes, before adding the cabbage and cooking for a further 15 minutes. Transfer the meat to a board, sprinkle over the oregano and leave to rest for a few minutes. Return the cooked veg to the broth and bring back to the boil to warm through. Divide the veg among your bowls and top with the broth. Slice the beef and add to the bowls with the celery leaves.